As a magnetic recording medium which is improved in properties by, as shown in FIG. 4, providing a preliminary layer 11 on a non-magnetic substrate 10 and laminating in order a primer layer 12 and a magnetic layer 13, there are a magnetic recording medium disclosed in Japanese Patent Laid-Open Publication No. 188732/1986 and a magnetic recording medium disclosed in Japanese Patent Laid-Open Publication No. 222021/1986.
The former is that wherein a non-magnetic, hard layer comprising an alumite layer is provided as the preliminary layer 11, having magnetic properties improved (crystalline orientating property) and a mechanical reliability improved. The latter is that wherein ZrO.sub.2, In.sub.2 O.sub.3 or SnO.sub.2 film is formed to improve coercive force or residual magnetic flux density.
Meanwhile, recently, a higher density magnetic recording medium has been vigorously developed. To obtain a high density one, a recording medium is necessary which can keep stably narrow spacing between head and disc and has a high S/N (a ratio of signal to noise). Therefore, there is expected and used a magnetic recording medium comprising a substrate with mirror surface on which has no any projections or foreign materials (substrate having no texture treatment and having small surface-roughness), for example, a non-magnetic, mirror-surface substrate such as a glass substrate.
However, in the case that the non-magnetic, mirror-surface substrate such as the glass substrate is used, noise, particularly modulation noise tends to increase, causing decrease of S/N, disturbing a high density recording. For example, comparing, in the same sputtering condition, a medium wherein a substrate conventionally texture-treated is used with a medium wherein a substrate which is mirror-treated is used, the latter medium has a ratio of S/N deteriorated as large as about -7 dB.